A tire bead filler rubber requires higher stiffness and hardness when compared to rubber compounds in other parts in order to support a load of a vehicle transferred to a rim when the rim is installed in a tire. Also, the high rubber stiffness is required to prevent bead unseating that may be caused during car driving.
A method of increasing a content of an inorganic substance such as clay to enhance hardness and stiffness of a rubber is used to improve stiffness of a tire bead filler rubber. However, when the content of the clay is high in this way, the hardness of the rubber is enhanced, but rolling resistance performance of tires may be deteriorated due to increase of heat generated between the clay and the rubber, rubber blending characteristics may be degraded, and a tensile strength may be lowered. Also, the clay commonly used in the art has a low reinforcing effect on a rubber composition due to a low oil adsorption quantity thereof.
As another method to improve the stiffness of the tire bead filler rubber, a method using a novolac-type phenol-based resin or a modified phenol-based resin to improve stiffness and hardness of the rubber has also been proposed. Such a method has an advantage in that the stiffness and hardness of the rubber are improved, but it has also a drawback in that stiffness of the rubber may be degraded as stiffness of the resin is degraded at a high-temperature condition, that is, a driving condition after tires are mounted.
As an alternative to such a method, Korean Unexamined Patent Publication No. 2011-0071605 discloses that two types of carbon black having different physical properties are used in a raw rubber, and Korean Unexamined Patent Publication No. 2016-0077895 discloses that a thermoplastic elastomer is used in a raw rubber to promote improvement of stiffness of the rubber. Such methods have an advantage in that the stiffness is somewhat improved, but they do not yield satisfactory results. In particular, since a natural rubber is used in the rubber composition as disclosed in the patents, it is impossible to secure sufficient hardness of a finally manufactured bead filler due to low compatibility with a synthetic rubber added together with the natural rubber.